1. Field of the Invention
The present invention relates to a battery module, which is a plurality of batteries connected in series and joined in a straight line via connecting hardware.
2. Description of the Related Art
Battery modules that join rechargeable batteries in a straight-line configuration are used primarily in electrically driven vehicles such as the hybrid car. In this type of battery module, it is important to solidly connect batteries in a manner that has low electrical resistance. This is because large connection resistance not only reduces output, but also generates heat due to Joule-heating, resulting in further resistive loss that prevents effective power utilization. A battery module that is a plurality of batteries connected in series and joined in a straight line via connecting hardware is cited in Japanese Patent Laid-Open Publications H10-106533 (1998) and 2001-185103.
As shown in FIGS. 1 and 2, battery modules cited in these disclosures join batteries by spot welding connecting hardware 80. As shown in FIG. 2, connecting hardware 80 is made by press-forming sheet metal in the shape of a circular cylindrical section 84 around the outside of a circular disk 83. As shown in the cross-section view of FIG. 1, connecting hardware 80 has a first welding section 81, which is the circular disk 83, welded to the sealing cap 92 of a battery 90, and a second welding section 82, which is the circular cylindrical section 84, welded to the outside surface of the external case 91 of another battery 90. This connects adjacent batteries 90 in series and in a straight-line configuration.
In the battery modules described above, batteries 90 are connected in series via connecting hardware 80 according to the following steps.
(1) The first welding section 81 of the connecting hardware 80 is set on the sealing cap 92, which is an end-plane electrode of one battery 90.
(2) A pair of welding electrodes is pressed onto the first welding section 81 and welding current is passed through the welding electrodes to weld the first welding section 81 to the sealing cap 92.
(3) The bottom end of another battery 90 to be connected is inserted into the circular cylindrical section 84 of the connecting hardware 80.
(4) A pair of welding electrodes is pressed onto the second welding section 82, which is the circular cylindrical section 84, and welding current is passed through the welding electrodes to weld the second welding section 82 to the outer surface of the battery 90.
For the connecting hardware described above, there is no detrimental effect on the batteries from the step that welds the first welding section to the sealing cap of a battery. This is because no electrode comes in close connection with the inside surface of the sealing cap. However, in the step that welds the second welding section to the outer surface of a battery, damage from the flow of high welding current can detrimentally affect that battery. This is because an internal battery electrode is in close connection with the inside of the outer surface of the battery where the second welding section is welded. As shown in FIG. 3, when the second welding section 82 is welded to the outer surface of the battery 90, the outer surface of the battery 90 is heated by welding heat. Since the internal electrode 93 is in close connection with this region, the heated outer surface can apply thermal stress and damage the internal electrode 93. In an application such as a hybrid car that has many batteries connected in series, failure of one battery can render all batteries useless. Consequently, it is extremely important to minimize battery failure during use. Therefore, during battery module manufacture, it is extremely important to reduce damage due to welding current as much as possible.
Further, as shown in the cross-section view of FIG. 4, connecting hardware 70 has been developed that welds a first welding section 71 and a second welding section 72 to opposing end-plane electrodes of the batteries 90. As shown by the arrow of FIG. 4, welding current flowing through this connecting hardware 70 simultaneously welds the first welding section 71 and the second welding section 72. To weld the first and the second welding sections to their respective end-plane electrodes in this type of battery module, it is necessary to apply a high welding current through the inside of the battery, and in particular, through battery electrodes. Consequently, high welding current can be the cause of electrode damage.
The present invention was developed to resolve the prior-art drawbacks. Thus, it is a primary object of the present invention to provide a battery module that can reliably prevent battery damage due to welding current.